1. Field of the Invention
This invention relates generally to trailer construction, and more specifically relates to an improved "drop frame trailer," which can either be configured to carry a single heavily-loaded container, or alternatively, can be configured to carry two unloaded containers.
2. Prior Art
Various forms of vehicle-towed trailers presently being manufactured are referred to as "flats," in that they define a raised upper flat loading surface, while other trailers are termed "drop frame trailers." In "drop frame trailers" the main load-receiving portion between the front and rear ends of the trailer is lowered relative to the front trailer end which overlies the wheel axles of the towing vehicle.
"Flats" are conventionally utilized for carrying various forms of bulk equipment and supplies, whereas drop frame trailers are utilized primarily to carry heavy loads which cannot be readily lifted to the high level of the load carrying surface of flats and require a lower center of gravity.
Still further, with the advent of containerization, marine shipping modules (usually referred to in the industry as "intermodal tank containers") were standardized in predominantly, twenty foot and forty foot lengths. The interface between the container modules and the securement system to the trailer were also standardized--a typical securement system utilizes twistlocks to secure the container at its corners to the trailer. This led to the development of trailers to carry containers of various sizes. The most common types of trailers were short trailers to carry a single twenty foot container, long trailers to carry a single forty foot container, and another type long trailer designed to carry either two twenty foot containers or a single forty foot container.
This concept then allowed the development of tank containers capable of carrying liquids. Due to the density of liquids typically transported, only a twenty foot module container could be used. If this twenty foot container is placed on a twenty foot trailer, the weight exceeds the legal over-the-road weight limitations on the trailer. In order to transport this container, it was thus required to utilize a longer trailer. Due to the heavy weight of such container, it was also highly desirable to utilize a drop-frame trailer to transport such containers.
Such drop frame trailers, in many instances, carry loads in only one direction due to the unavailability of a return load of the same type being available. Furthermore, in many instances, when a drop frame trailer is utilized to carry a load in one direction, the only load which may be available therefore in the return direction, may be the empty container utilized to carry the heavy load. It is therefore wasteful of time, money, and effort to convey only a single empty container for a trailer which is of a size sufficient to carry more than one empty container, but which is unsuitable for carrying such a plurality of containers. Accordingly, a need exists for an improved form of a drop frame trailer, which may be readily converted to carry a plurality of empty containers.